Why does changing animation layer make my model very small?
If you open the action called Genesis2Female|Animation|Base Layer you can see that you've keyframed the XYZ scale of the armature itself:

You can see the keyframed scale in the N panel &gt; Item &gt; Transform, it has been set at 0.01:

When you open your file, we can see that the armature is set to a default scale of 1/1/1, so as soon as you select this action, the armature is scaled down to 0.01 of its size.
To fix that, select the action, select the 3 Scale tracks and press X.
The little bit confusing thing is that the armature transforms (location, rotation and scale) are part of an action, not only the bone keyframes.